The present invention relates to a colour cathode ray tube of the shadow mask type.
The cathodoluminescent screens of such cathode ray tubes frequently comprise triplets of vertically extending stripes of phosphors which luminesce in different colours. The shadow mask is a metal sheet having columns of elongate apertures, contiguous apertures in each column being separated heightwise by a bridge. Also the apertures in adjacent columns are staggered by half the vertical pitch of the apertures at the central portion of the shadow mask. This staggering of the columns of apertures reduces moire effects in the image as viewed and also provides the mask with a greater rigidity than would be the case if the columns were not staggered. However when making the cathodoluminescent screen by a photographic process, such as that disclosed in British Patent Specification 1501549 the shadow mask is used as a master through which light is projected to expose photoresist applied to the faceplate. As the top and bottom edges of the mask hole pattern are curved, the border line will intersect some of the holes. In consequence the remaining parts of some apertures are so small that during screen manufacture there is either a very small phosphor dot or no phosphor dot on the screen. In the case of a column terminating in a bridge or part of a bridge then the phosphor stripe stops short of the nominal screen edge. Both these shortcomings manifest themselves in fairly rapid variations in the picture edge of the screen.
There have been many proposals for producing a smooth picture edges north (top) and south (bottom). One of these is European Patent Specification 0 001 673-A2 in which all the columns terminate in complete apertures having the same horizontal width as that of the nearby apertures in the same column. In order to ensure that all the columns terminate in complete apertures then (1) in those cases where a column would otherwise be terminated by a shorter than normal aperture, the short aperture is joined to the penultimate in the column by eliminating the bridge therebetween, and (2) in those cases where the column terminates in a bridge then the adjacent slot is lengthened by removal of the bridge material. Whilst such a proposal will produce a smooth edge it has the drawback that the mask transmission through these longer than normal apertures is somewhat larger than is intended and results in a sudden and unacceptable variation in brightness.